The present invention is concerned with the manufacture of a composite packing material by means of a solventless adhesive and, more particularly, is concerned with the manufacture of a material containing at least one film of plastics material.
In the present invention, there is to be understood by composite, or more simply "laminated," packing material, the material obtained by bonding together at least one film, preferably of plastics material, and at least one film of any other material, which may be plastics material, metal, paper, fabric or the like.
By solventless adhesive, there is understood an adhesive the use of which for a bonding operation does not entail the evaporation of volatile materials. This excludes, in particular, water-based glues, glues in the form of a dispersion, solution or emulsion and also glues constituted by a natural or synthetic resin dissolved or emulsified in at least one volatile organic solvent. A typical representative of a solventless adhesive which is preferably used according to the present invention is that known as a two (or multi-) reactive component adhesive, the two (or more) components of which react together in situ to form a cross-linked adhesive polymer. Examples of this kind of adhesive include a hydroxylated polyester or polyether which is reacted with a di- or polyisocyanate and an expoxy resin which is reacted with compounds containing at least two active hydrogen atoms. It is to be understood that any solventless adhesive as defined above may be used for carrying out the present invention and that the nature of this adhesive is not limited in any way to the examples given above solely by way of illustration.
At present, laminates of the kind defined above are manufactured exclusively with solvent type adhesives; since these contain, as bonding agent, a high molecular weight polymer which has already been formed, the initial bond is sufficient to produce a firm joint between the films bonded together. Furthermore, the solvent makes it possible to adjust, as desired, the viscosity best suited to the application of the adhesive to the film and also to adjust the weight of adhesive applied per unit of area to values which are as low as desired. Nevertheless, this technique suffers from numerous disadvantages. First, for obvious reasons of hygiene, it is necessary to provide special, very expensive installations to prevent the poisoning of operators by the solvent vapors and to avoid all risk of fire. In addition, the use of a solvent for the preparation of the adhesive entails various stages of operation, such as dissolving the adhesive in the solvent, intimate mixing of the components and, finally, the elimination of the solvent, preferably in an oven, before bonding the laminate, which entails great expenditure of time and cost of equipment. In these operations, either the solvent is lost, which represents a dry loss of raw material, or else it is recovered as far as possible but this also requires expensive installations. Another disadvantage is that the active life of the adhesive in its ready for use form is limited, this life being inversely proportional to the reactivity of its components. If the machine manufacturing the laminates should stop, the adhesive still remaining in the coating vessels is lost, because its period of reactivity or "pan life" is too limited.
It would be much more advantageous to use solventless adhesives, because the disadvantages inherent in the use of solvents would be eliminated. Nevertheless, it has not hitherto been found possible to use solventless adhesives in the manufacture of laminates for the following reasons: the initial adhesive power of a solventless adhesive is very low because the polymerization (polyaddition) of its components does not start until the moment when the adhesive is applied to the sheet of laminate to be manufactured and, consequently, adhesivity, which is a corollary of polymerization (polyaddition), is acquired only after a certain time, which may vary from a few minutes to several hours. This low initial molecular weight of the solventless adhesive is, nevertheless, necessary in order to prevent the system from acquiring excessive viscosity which would prevent the application of the adhesive to the laminate sheet at ambient temperature or at higher temperature. This low viscosity is all the more necessary because, in the manufacture of laminates, an extremely thin layer of adhesive is applied, generally with a thickness of from 0.5 - 5 microns.
It will be readily appreciated that it is extremely difficult to spread a layer of adhesive at a rate of 0.5 to 5 grams per square meter (a thickness of 0.5 to 5 microns) uniformly on a sheet of plastics material. It is a solution of this problem, which hitherto has constituted an obstacle to the use of a solventless adhesive in the manufacture of laminates, which constitutes the object of the present invention.
There is already known a system for the application of a solventless adhesive to a sheet of plastics material (see British patent specification No. 1,158,740) but the method and apparatus described in this patent specification do not permit the manufacture of laminates. This patent specification contemplates, in particular, the application of a strip of adhesive to a sheet of plastics material for the purpose of manufacturing bags. This is clear from the accompanying drawing, in which a stereotype cylinder 5 applying adhesive to a sheet of plastics material has the dimensions required for forming a narrow adhesive strip. It would obviously be possible to conceive this cylinder 5 as having a suitable width for applying the adhesive over the entire width of a sheet of plastics material, for example with a width of 1.20 meter but this would implicitly entail a corresponding increase of the dimensions of the two-cylinder system (counter-cylinder 2 and applicator cylinder 1 ) used for feeding the adhesive to the sterotype cylinder 5. Although this patent specification does not specify this, the counter-cylinder 2 and the applicator cylinder 1 are necessarily of metal because each of them contains an internal heating system 14 and 15 and, consequently, they must be good conductors of heat. In order to be able to apply such small amounts of adhesive of 0.5 - 5 grams per square meter, preferably 1 to 2.5 grams per square meters, it would be necessary to bring the counter-cylinder 2 so close to the feeder cylinder 1 that seizing would become inevitable because of the large dimensions which they would need to have. This is all the more so because, according to this patent specification, all these cylinders have different circumferential speeds, the ratio of circumferential speeds between the applicator cylinder 1 and the counter-cylinder 2 being from 1.1 to 5, preferably, from 1.5 to 1.7, and the ratio of the circumferential speeds between the stereotype cylinder and the applicator cylinder being from 1 to 3.5. These different circumferential speeds, which constitute an essential characteristic of this patent specification, have the object of producing shearing forces effecting intimate mixing of the old adhesive, which is still contained on each of the cylinders, with the fresh adhesive supplied, in order to avoid the formation of "cobwebs." For the purpose of lamination, it is necessary to prevent old adhesive, the degree of condensation of which has already reached too advanced a stage, from remaining on the rollers, as this would entail difficulties in the uniform transmission of the complete layer of adhesive to the sheet to be coated and would cause the sheet of plastics material to stick to the stereotype roller. Precisely the fact that the apparatus of this patent specification necessarily contains a stereotype cylinder has the consequence that there will always be a mixture of fresh adhesive and old adhesive, because the depressions in the stereotype roller become progressively filled with hardened adhesive, which, after a certain period of operation, causes a film of plastics material to stick to the stereotype cylinder.